Along with the recent spread of portable electronic devices such as a portable personal computer, a handy video camera, and an information terminal, a nonaqueous electrolyte secondary battery having a high voltage and a high energy density is widely used as a power supply. Battery cars and hybrid cars in which electric power is used in a part of the motive power have been put into practical use in terms of environmental problems.
Negative and positive electrodes of the nonaqueous electrolyte secondary battery are produced by coating, on a metal collector, a slurry or pasty coating liquid, produced by mixing an electrode active material, an electroconductive material, a binder (binding agent) and a solvent. As such a solvent used in the positive or negative electrode, an organic solvent such as dimethylacetamide, acetone, dimethylformamide, and N-methylpyrrolidone or water is used; however, since the organic solvent imposes high load on the environment, water is often used as the solvent in recent years.
As a binder or a thickener used when a solvent is water, there is used a polymeric carboxylic compound such as polyacrylic acid and carboxymethylcellulose; polyvinyl alcohol, polyethyleneglycol, carboxymethylcellulose, polyvinylpyrrolidone, polyacrylic acid amide, or the like.
When the polymeric carboxylic compound is used, there has been known that a carboxyl group thereof is adsorbed to a surface of a metal collector, an electrode active material, an electroconductive material, or the like, whereby a good electrode excellent in binding properties can be obtained (for example, see Patent Literature 1).
However, in a nonaqueous electrolyte secondary battery having a negative electrode using a polymeric carboxylic compound, there is a problem that an irreversible capacity is easily increased at the time of initial charging, and there is a problem that the electrical capacitance is easily reduced or an internal resistance is easily increased by storage at high temperature or repetition of discharge and charge at high temperature. Such a problem often occurs particularly in a nonaqueous electrolyte secondary battery having a negative electrode containing a polymeric carboxylic compound and a crystalline carbon material such as graphite.
It is considered that this is because the polymeric carboxylic compound is decomposed at an active site on the negative electrode, and particularly an end surface of a highly reactive crystalline carbon material, and a current is consumed at this time, whereby an irreversible capacity is generated at the time of initial charge. The decomposition product in a high temperature state further reacts with an electrolyte or the like, whereby the irreversible capacity is increased, and, at the same time, a large amount of decomposition products is accumulated to cause inhibition of movement of lithium ions on a negative electrode surface. When the positive electrode is one containing a nickel compound or an iron compound as an active material, the extent of such inhibition may be further increased.
Meanwhile, in the nonaqueous electrolyte secondary battery, various additives for nonaqueous electrolyte solutions are proposed to enhance the stability and the electrical characteristics, and additives such as 1,3-propanesultone (for example, see Patent Literature 2), vinyl ethylene carbonate (for example, see Patent Literature 3), vinylene carbonate (for example, see Patent Literature 4), 1,3-propanesultone, butanesultone (for example, see Patent Literature 5), vinylene carbonate (for example, see Patent Literature 6), and vinyl ethylene carbonate (for example, see Patent Literature 7) form a stable coat called SEI (Solid Electrolyte Interface) on a surface of a negative electrode, and it is considered that reductive decomposition of the nonaqueous electrolyte solution is suppressed by coating the negative electrode surface with the coat. Among those additives, vinylene carbonate is widely used because it is considerably effective.
However, although the above additives exhibit a certain effect in reduction of the irreversible capacity at the time of initial charge in a negative electrode containing a polymeric carboxylic compound, it cannot be said that the effect is sufficient. Since the above additives in a high temperature state has a high reactivity with a decomposition product of the polymeric carboxylic compound and forms a thick coat, a sufficient protective effect against a reduction of the electrical capacitance and an increase of the internal resistance caused by storage at high temperature or repetition of discharge and charge at high temperature cannot be obtained.
Although the polymeric carboxylic compound is sometimes used by being neutralized or partially neutralized with amines or alkali metal in order to enhance the water solubility and regulate pH, the polymeric carboxylic compound in this invention includes such neutralized product and partially neutralized product of the polymeric carboxylic compound.